This post will discuss how to reverse mappings of a map in JavaScript.

Reversing mappings of a map in JavaScript means creating a new map that has the keys and values swapped, so that the original values become the new keys and the original keys become the new values. This can be useful if we want to look up the keys by their values, or if we want to reverse the order of the map entries. However, the original map must have unique and valid values, otherwise some keys will be lost in the inversion process. For example, if the original map has two entries with the same value, only one of them will be preserved in the reverse map. Here are some common functions to invert a map:

1. Using a for…of loop

This is a simple and concise way to loop through the original map and create a new map with swapped keys and values. We can use the Map.entries() function to get an iterator object that returns an array of [key, value] pairs for each element in the original map. Then, we can use array destructuring to assign the key and value variables and swap them inside the loop. The following code illustrates this:

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2. Using forEach() function

This is another way to loop through the original map and execute a callback function for each element. The callback function receives the value, key, and the original map as arguments. We can use these arguments to set the swapped entries in the new map. The following code illustrates this:

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3. Using Array.from() and reverse() functions

This is a way to create a new map from an array of reversed key-value pairs from the original map. We can use the Array.from() function to convert the original map into an array of [key, value] pairs with each pair reversed in place. Then we can simply pass this array to the Map() constructor. The following code illustrates this:

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Note that all these functions assume that there are no duplicate values in the original map, otherwise some keys will be lost in the reverse map. The original values must be valid keys for the new map, otherwise an error will be thrown. Also, these functions do not preserve the order of the original map.